Modern concepts in industrial automation, that is to say the control and monitoring of technical processes with the aid of software, are based on the idea of a central controller with a distributed sensor/actuator level. In this case, the subscribers communicate with one another and with superordinate systems via industrial data networks, also referred to as automation networks below.
Ethernet is the most widespread communication standard in local area networks and is stipulated primarily by the IEEE standard 802.3. Ethernet is based on a LAN structure in which a plurality of control nodes, for example computers or machines, are connected to one another in a wired manner, wherein the Ethernet protocol encapsulates the data to be transmitted in data packets, also referred to as a message below, with a predetermined format. In this case, it is possible to use different Ethernet variants which differ in terms of the transmission rate, the types of cable used and the line coding.
Communication between the subscribers generally takes place via a data line having four wires in industrially used Ethernet networks, wherein the four wires are often in the form of two twisted pairs of wires. Twisting the pairs of wires reduces crosstalk. The two wires in a pair of wires are always used together, wherein a differential data signal is transmitted via one pair of wires in each case. Sometimes, all four wires are also twisted together, which has the advantage of a higher degree of flexibility of the line.
An often compulsory requirement imposed on automation networks is that the outputs of a machine, the actuators, can be moved to a safe state at any time without losing the possibility of monitoring the machine. The sensors and the controller in the automation network must therefore be able to be operated independently of the actuators. For this reason, the energy supply for the actuators is generally separate from the energy supply for the sensors and the controller in order to disconnect the actuator supply independently of the sensor and controller supply.
The energy supply lines required in the automation network are generally laid independently of the data line, which makes it necessary to have at least two dedicated cabling systems with the corresponding contact technology. However, in industrial automation, it is desirable to always achieve cabling which is as cost-effective and simple as possible in the automation network. This is important, in particular, when good shielding, a high protection class or a high temperature resistance is required for the cabling on account of environmental requirements. The cabling is therefore often responsible for a relevant high proportion of the system costs.
One approach to saving costs in respect of cabling is to combine the power supply and the data transmission in one cabling system. With the aid of the so-called “Power over Ethernet” standard, an individual voltage can therefore be concomitantly transmitted in addition to the two differential data signals on the two twisted pairs of wires using a four-wire standard Ethernet data line. A specially adapted Ethernet transformer is used for this purpose in the PoE standard, which transformer applies the two potentials required for the voltage to each pair of wires via a center tap. However, since two separate energy supplies are often required in industrial automation, one for the actuators and the other for the sensors and the controller, an additional cabling system for a second energy supply is also required in a PoE system having a four-wire data line.